In many applications it is desirable to automatically control the level of an audio signal. Traditionally, such an audio signal processor is known as an automatic gain control (AGC), compressor, or limiter. A special type of such signal processors contains a (simple) estimator of loudness level, such that the processor attempts to control the level in a way that corresponds to the perceived loudness of the signal. Certain existing processors can perform level detection based on an RMS level—which may be considered a primitive loudness level estimator.
Some pre-existing dynamic range compressors can operate in real-time. That is, the processed (output) signal is generated ‘immediately’, typically meaning with a delay—due to the processing itself and the platform on which it is implemented—of a few seconds or less. Such processors have no knowledge or ‘model’ of the content which they are processing. By setting the controls of such processors, the user must specify directly the parameters of the actual processing. For instance, a “threshold” parameter is typically used to specify an absolute level, above which the material should be compressed, and a “ratio” parameter specifies how much the levels above the threshold should be compressed, and finally a “makeup gain” specifies a gain to be applied, in order to amplify the output level by a predetermined amount. All such parameters describe the specific processing to be performed—not taking into account any property of the actual content to be processed.
FIG. 1 shows the blocks diagram of a typical prior art dynamic-range compressor. In Zölzer's “DAFx−Digital Audio Effects” (Wiley, 2011) the technical details of several variations of such processors are provided. In FIG. 1 the Level detector block (104) may implement a 1st order IIR-based RMS measurement (Zölzer, FIG. 4.10). The Delay block (101) provides so-called look-ahead delay, which serves to time-align the audio signal with the control signal from the side-chain. The Gain control block (105) calculates an attenuation gain value, based on the output from the Level detector (104) and a given threshold level and compression ratio (106). The Attenuator block (102) then multiplies the signal amplitude with the attenuation gain.
A different type of pre-existing dynamic range compressors operate according to the “2-pass method”, and can process stored content such as sound files. Such processors first analyze the content (pass 1), and measure various properties of the material, such as overall level and dynamic range (or level “spread”). After completing pass 1, such processors will then carry out pass 2, in which they will perform the actual processing, and typically generate an output sound file. In contrast to the pre-existing real-time processors, the 2-pass processors may take into account various properties of the content (measured in pass 1), while setting up the processing (taking place in pass 2). Thus, the parameters of such processors may specify desired properties of the processed (output) signal, rather than properties of the processing itself. For example, a target loudness level may be specified, and the 2-pass processor would then set, for example, its “makeup gain” such that the overall loudness level of the processed sound file would match the specified target.
Loudness control processing has become increasingly relevant in the past decade. In broadcasts in radio/TV and other media, regulations may require that the programme itself—or commercials within/between programmes—must not exceed a certain loudness level and/or range. Both international and national standards and recommendations have been published in recent years, specifying and supporting such regulations, by organizations including ITU-R, EBU, ATSC, and BCAP.
The only ways to comply with such regulations have involved storing the entire programme as a file, and using a 2-pass processor. Even such processors are typically capable of matching only the target loudness level—not a target loudness level and a target loudness range. Or alternatively, real-time processing might be required, such as in a live production or broadcast transmission. In this case, a skilled operator would need to be present to setup the processing parameters according to his knowledge of the material to be processed. He would then often use a loudness meter, in order to check properties of the processed signal, and possibly adjust the processing parameters. In any case a time-consuming process and/or an extended work-flow.